Powering Up: Temporary Generators

While preparing a presentation, I noticed a photo taken at a facility where temporary power was needed periodically. The facility rented a generator when necessary. It showed a disconnecting means with four individual conductors stubbed out of the bottom of the enclosure. The electrical contractor had installed the disconnecting means immediately inside the building in a dry location where a portable generator could be connected to it with individual cables. A manual transfer switch was located downstream from the disconnecting means, which would indicate the application was an optional standby load, rather than an emergency or legally required standby load, since both require automatic transfer upon loss of normal power. The installation raised a number of questions concerning the proper application of the National Electrical Code.

The most obvious question is whether individual cables are permitted to be installed from the generator to the first disconnecting means. Based on the exception in 590.4(B), covering feeders in temporary installations, single insulated conductors are permitted to be installed for temporary power and lighting during emergencies but only where physical damage is not an issue. If the owner and the authority having jurisdiction both agree the generator is a necessity and that power and lighting is deemed an emergency, individual conductors could be installed for this application.

The next question is how the individual conductors can be connected to the generator and to the disconnecting means. At both the generator and the disconnecting means, a suitable connector must be installed to permit the cables or conductors to enter into the generator enclosure and the disconnecting means enclosure. The combined size of the five cables or conductors (three ungrounded conductors, one grounded or neutral conductor, and an equipment-grounding conductor) is probably too large for a single gland nut or cable connector, and other fittings, such as chase nipples or similar fittings, are not appropriate for this application.

At the facility in question, the electrical contractor installed single-pole separable connectors (commonly called cam-lock systems) at the disconnecting means similar to the method permitted in Article 520 for portable switchboards in theaters, motion picture and television studios. Section 520.53 permits this application, but there are many safety features that must be provided before this method can be used.

The installation of a cam-lock connector system requires qualified personnel, extra-hard usage cords or cables, 2 AWG and larger cables, at least one 6 AWG equipment-grounding conductor, and as other safety features. In addition, these systems require compliance with at least one of the following conditions:

1. Connection and disconnection of connectors are permissible only where the supply connectors are interlocked to the source (the generator), and it is not possible to connect or disconnect connectors while the supply is energized. (This condition never would be used for a generator installation as described in the above introductory paragraphs.)

2. Line connectors must be of the listed sequential-interlocking type so that load connectors only can be connected in the following sequence: (a) the equipment-grounding conductor connection first; (b) the grounded conductor connection next, where used; (c) and then the ungrounded conductor connections; (d) and the disconnection must be in the reverse order. Only a very experienced person, working with these systems on a regular basis, would know how to connect and disconnect the cam-locks for the generator power.

3. A caution notice must be provided and located adjacent to the line connectors (at the disconnecting means) indicating that the plug connections must be in the following order: (a) the equipment-grounding conductor connection first; (b) the grounded conductor connection next, where used; (c) and then the ungrounded conductor connections; (d) and the disconnection must be in the reverse order. (There wasn’t a caution notice provided at the disconnecting means in the photo indicating a lack of understanding of the connection and disconnection sequence of the generator conductors.) The connection and disconnection sequence always will leave the neutral in the circuit (where used) until all phase conductors are connected or disconnected to ensure an open-neutral imbalance does not occur. The equipment-grounding conductor always must be the first connected and the last disconnected to ensure a ground fault path in case of a fault.

Finally, the bottom of the disconnecting means must be a nonferrous metal (aluminum), based on the requirements in 300.20(B), for each conductor to be individually connected, so there would be no hysteresis heating of the metal from currents in the individual conductors. There was no indication the disconnecting means enclosure in the photo was aluminum. Be careful to comply with all of the requirements mentioned when installing generators.

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 and mark.c.ode@us.ul.com.

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